U.S. patent application number 12/477993 was filed with the patent office on 2010-12-09 for method for frost protection in a direct methanol fuel cell.
Invention is credited to Madeleine Odgaard.
Application Number | 20100310954 12/477993 |
Document ID | / |
Family ID | 43300987 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100310954 |
Kind Code |
A1 |
Odgaard; Madeleine |
December 9, 2010 |
Method for frost protection in a direct methanol fuel cell
Abstract
Systems and methods for protecting fuel cell systems from frost
by introduction of a freezing point depressant into the fuel cell
system and/or flushing the fuel cell system with an insert gas are
provided.
Inventors: |
Odgaard; Madeleine; (Odense
M, DK) |
Correspondence
Address: |
Licata & Tyrrell P.C.
66 E. Main Street
Marlton
NJ
08053
US
|
Family ID: |
43300987 |
Appl. No.: |
12/477993 |
Filed: |
June 4, 2009 |
Current U.S.
Class: |
429/429 ;
429/452 |
Current CPC
Class: |
H01M 8/04186 20130101;
H01M 8/04253 20130101; Y02E 60/523 20130101; H01M 8/1011 20130101;
H01M 8/04955 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/429 ;
429/452 |
International
Class: |
H01M 8/00 20060101
H01M008/00; H01M 2/00 20060101 H01M002/00 |
Claims
1. A fuel cell system comprising a fuel cell stack supplied with a
liquid methanol/water fuel mixture on the anode side and air on the
cathode side, a methanol storage tank, a water storage tank, a
circulation pump for circulating the liquid methanol/water fuel
mixture from the methanol and water storage tanks through the fuel
cell stack and the partially depleted methanol/water mixture to a
storage tank, and a freezing point depressant storage tank which
supplies the fuel cell stack with a freezing point depressant
during shut-down.
2. The fuel cell system of claim 1 wherein the freezing point
depressant tank contains an alcohol which diffuses at a slower rate
through a proton exchange membrane than methanol.
3. The fuel cell system of claim 2 wherein the alcohol is a normal,
branched aliphatic or cyclic structural isomer of ethanol,
propanol, butanol, pentanol, hexanol, heptanol or octanol, or a
mixture thereof.
4. The fuel cell system of claim 1 wherein the freezing point
depressant tank contains a liquid glycol with a carbon content
greater than two carbon atoms in the molecule.
5. The fuel cell system of claim 4 wherein the glycol is a normal,
branched aliphatic or cyclic structural isomer of ethanediol,
propandiol, butandiol, pentandiol, hexandiol, heptandiol or
octandiol, or a mixture thereof.
6. The fuel cell system of claim 1 wherein the freezing point
depressant tank contains an anti-freeze mixture that has a
water/alcohol ratio, or a water/glycol ratio that ensures that the
mixture remains liquid at temperatures in the range +80.degree. C.
to -40.degree. C. and prevents freezing of the solution at
temperatures in the range 0 to -50.degree. C.
7. A method for frost protecting a fuel cell system during
shut-down comprising: releasing the electrical load on a direct
methanol fuel cell (DMFC) and bringing the DMFC to an open circuit
state; interrupting the feeding of methanol fuel to a liquid fuel
compartment of a DMFC; and replacing any methanol in the DMFC with
a freezing point depressant.
8. The method of claim 7 wherein the freezing point depressant
comprises an alcohol which diffuses at a slower rate through a
proton exchange membrane than methanol.
9. The method of claim 8 wherein the alcohol is a normal, branched
aliphatic or cyclic structural isomer of ethanol, propanol,
butanol, pentanol, hexanol, heptanol or octanol, or a mixture
thereof.
10. The method of claim 7 wherein the freezing point depressant
comprises a liquid glycol with a carbon content greater than two
carbon atoms in the molecule.
11. The method of claim 10 wherein the glycol is a normal, branched
aliphatic or cyclic structural isomer of ethanediol, propandiol,
butandiol, pentandiol, hexandiol, heptandiol or octandiol, or a
mixture thereof.
12. The method of claim 7 wherein the freezing point depressant
comprises an anti-freeze mixture that has a water/alcohol ratio, or
a water/glycol ratio that ensures that the mixture remains liquid
at temperatures in the range +80.degree. C. to -40.degree. C. and
prevents freezing of the solution at temperatures in the range 0 to
-50.degree. C.
13. A fuel cell system comprising a fuel cell stack supplied with a
liquid methanol/water fuel mixture on the anode side and air on the
cathode side, a methanol storage tank, a water storage tank, a
circulation pump for circulating the liquid methanol/water fuel
mixture from the methanol and water storage tanks through the fuel
cell stack and the partially depleted methanol/water mixture to a
storage tank, and an inert gas storage tank which flushes the fuel
cell stack with an inert gas during shut-down.
14. The fuel cell system of claim 13 wherein the inert gas storage
tank contains an inert gas selected from nitrogen or carbon dioxide
produced by the fuel cell chemical reaction.
15. The fuel cell system of claim 13 wherein the inert gas storage
tank contains carbon dioxide separated as a by-product from liquid
fuel of the fuel cell during operation of the fuel cell system.
16. The fuel system of claim 13 further comprising a freezing point
depressant storage tank which supplies the fuel cell stack with a
freezing point depressant during shut-down.
17. A method for frost protecting a fuel cell system during
shut-down comprising: releasing the electrical load on a direct
methanol fuel cell (DMFC) and bringing the DMFC to an open circuit
state; and flushing the fuel cell system with an inert gas which
does not react with methanol in the fuel cell.
18. The method of claim 17 wherein the inert gas is nitrogen or
carbon dioxide.
19. The method of claim 17 wherein the inert gas is carbon dioxide
separated as a by-product from liquid fuel of the fuel cell during
operation of the fuel cell system.
Description
[0001] This patent application claims the benefit of priority from
U.S. Provisional Application Ser. No. 60/058,617, filed Jun. 4,
2008, teachings of which are herein incorporated by reference in
their entirety.
FIELD OF THE INVENTION
[0002] This invention relates to a method of reducing degradation
of direct methanol fuel cell (DMFC) components at temperatures
below room temperature and especially below the freezing point of
fuel/water mixtures.
BACKGROUND OF THE INVENTION
[0003] In both stationary and portable power applications, fuel
cell systems are required to operate for varying time periods and
may be subjected to frequent start-up and shut down cycles.
[0004] After shut down, direct methanol fuel cells (DMFC) must be
re-started in order to provide sufficient power for the application
to be supplied with power. The power generated is provided for
running any arbitrary electrical prime mover or other device that
consumes electric power that is electrically connected to the power
generating fuel cell. This configuration is termed the
application.
[0005] At ambient temperatures above freezing, re-starting
generally occurs without problems. However, lower temperatures slow
down the re-activation process and when the ambient temperature
falls below the freezing point of the methanol/water mixture in the
DMFC, the re-starting process is complicated by the effects of
phase change as well as being markedly more sluggish.
[0006] Repeated freezing and thawing of the water in the DMFC
gradually degrades cell performance and efficiency of the DMFC
(McDonald et al. Fuel Cells 2004 4(3) pp 208-213). It is therefore
desirable to ensure that the aqueous liquid/fuel mixtures contained
in the fuel cell remains liquid at temperatures below the freezing
point of the mixture. Provisions must be made to prevent damage to
the fuel-cell when the methanol and water mixture freezes.
[0007] When a fuel cell is dormant, methanol diffuses from the fuel
stock adjacent to the anode to the cathode side and then reacts
with residual oxygen. This reaction has been observed to damage and
degrade the Membrane Electrode Assembly (MEA) resulting in
deterioration of the fuel cell performance. Further, when
restarting the DMFC from the dormant state, methanol and oxygen on
the cathode side, in contact with the electrode catalyst, react and
cause local overheating of the fuel cell with resulting degradation
of the electrode assembly materials and deterioration of DMFC
performance.
[0008] Several methods have been suggested for protection and start
up of fuel cells at temperatures below 0.degree. C. These include:
adding a heater element from an external source to maintain
temperatures above the freezing point; insulation; drainage of
fluid to avoid ice formation by using an ancillary supply of inert
gas; providing separated cooling/heating circuits; and promoting
start up by pre-heating the fuel cell to a temperature above the
freezing point of the standard methanol/water fuel mixture prior to
replacing a suitable storage liquid, which substitutes methanol and
water on shut-down, with the fuel mixture.
[0009] However, active heating expends energy which is considered
as a net loss of output and reduces efficiency. Further, many of
these solutions increase the complexity of operation and the
complexity of the system components. They increase installation
cost and/or they require an energy supply provided from an external
source.
[0010] U.S. Pat. No. 6,440,595 discloses a fuel cell system
comprising a fuel cell which includes a feed line for a fuel and a
feed line for an oxidant. To ensure adequate moistening of the fuel
cell membrane even during the start-up phase of the fuel cell, a
fluid reservoir containing a fluid is provided, via which the fuel
and/or the oxidant are humidified before entering the fuel cell.
Thus, adequate moistening of the fuel cell membrane is ensured even
during the start-up phase of the fuel cell. To prevent the fluid
from freezing at low temperatures, the fluid is mixed with an
antifreeze. To ensure that the antifreeze will not pass into the
fuel cell, the fluid drawn from the fluid reservoir is heated
sufficiently by a heating means for evaporation of the antifreeze
and separation from the fluid to take place.
[0011] U.S. Pat. No. 6,905,791 discloses a method and apparatus for
the operation of a fuel cell system to avoid the freezing of water
residing in one or more fuel cells during periods of system
inactivity wherein a chemical compound is introduced into the fuel
cell system to mix with the water still resident within the fuel
cell thereby lowering the temperature upon which the onset of water
freezing occurs. The system is configured such that the chemical
compound can be introduced at numerous different locations. The
chemical compound is taught to be in a liquid state at the normal
freezing temperature of the reaction product (which in the case of
water is approximately 32.degree. F.) but in a gaseous state at
temperatures corresponding to the fuel cell system when the system
is at its normal operating condition. The chemical compound is
preferably miscible in water, and has a boiling point in the range
between approximately 68.degree. F., and approximately 176.degree.
F., with a freezing point below approximately -40.degree. F.
Compounds especially adapted for use in this fuel cell system are
taught to include alcohols (such as methanol), bases, acids, sugars
with at least one functional group and one to twenty carbon atoms,
and compounds including hydrogen and at least one carbon and
nitrogen. The chemical compound should also be able to be
catalytically broken down in the fuel cell and the product
generated burned in a combustor and vented to the atmosphere. The
chemical compound can be introduced alone under pressure in the
vapor phase, or in combination with the fuel, the incoming oxygen
or an inert gas.
SUMMARY OF THE INVENTION
[0012] An aspect of the present invention relates to methods and
systems for adding to a fuel cell system a freezing point
depressant that is compatible with fuel cell material components
and that does not deleteriously affect electrode processes of the
fuel cell system.
[0013] Another aspect of the present invention relates to methods
and system which utilize an inert gas, preferably carbon dioxide
already present in the system as a reaction product generated upon
the oxidation of methanol, to flush the fuel cell system during
fuel-cell-shut-down.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a line graph showing freezing curves for various
mixtures of water and exemplary freezing point depressants. FIG. 1
shows the volume percentage needed for the exemplary freezing point
depressants to provide frost protection down to -40.degree. C.
[0015] FIG. 2a through 2c are diagrams of an exemplary fuel cell
system of the present invention with a freezing point depressant
tank and/or an inert gas tank. FIG. 2a shows an exemplary fuel cell
system with a freezing point depressant tank. FIG. 2b shows an
exemplary fuel cell system with an inert gas tank. FIG. 2c shows an
exemplary fuel cell system with a freezing point depressant tank
and an inert gas tank.
[0016] FIG. 3 is a line graph showing the development of current
and voltage of a DMFC, which has been stored at -30.degree. C.
filled with a 50 vol % propylene glycol/water mixture. As shown,
after initial activation, it is possible to draw current after only
8 minutes and a stable voltage is obtained after approximately 30
minutes
[0017] FIG. 4 is a line graph showing the development of current
and voltage of a DMFC, which has been stored at -20.degree. C.
filled with a 50 vol % ethanol/water solution. As shown, nominal
performance is obtained within 20 minutes after initiation.
[0018] FIG. 5 is a line graph showing the development of current
and voltage of a DMFC which has been stored overnight with a 30 vol
% methanol/water mixture at -20.degree. C. The fuel cell was stored
and started up without controlling the amount of oxygen present in
the DMFC leading to degradation of the membrane electrode
assembly.
[0019] FIG. 6 shows the development of current and voltage of a
DMFC, which has been stored at -18.degree. C. with a 50 vol %
ethanol/water mixture and a 50 vol % propylene glycol/water
mixture. Oxygen was removed by flushing using an inert gas. The
figure shows the current/voltage curve obtained before and after
storage. No degradation is observed.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention provides fuel cell deactivation
processes that leave the direct methanol fuel cell (DMFC) in a
non-reactive state during dormant periods and provides for
carefully controlled re-activation of the dormant cell without
adding complexity and without lowering efficiency of the fuel
cell.
[0021] In one embodiment of the present invention, a freezing point
depressant (FPD) is added to the fuel cell system prior to
deactivation. An apparent and obvious choice of a freezing point
depressant to be added to the fuel cell is methanol. However,
methanol diffuses very quickly in the proton exchange membrane
(PEM) where it oxidizes at the cathode and can participate in
undesirable side-reactions or electrolysis leading to irreversible
membrane electrode degradation. Accordingly, in the present
invention, freezing point depressants (FPDs) are selected which
exhibit a significant reduction in diffusion through the PEM by a
factor of 10 or more as compared to methanol. Thus, the FPDs
selected for use in the present invention comprise compatible
molecules that are substantially heavier and larger than methanol,
not degraded on contact with oxygen or air in the fuel cell and at
the same time are electrochemically active so that these molecules
can be used to re-activate the cell. Examples of FPDs for use in
the present invention include, but are not limited to, alcohols
such as ethanol, propanol, butanol, pentanol, hexanol, heptanol and
octanol including any normal, branched aliphatic or cyclic
structural isomers, and mixtures of these, which diffuse at a
slower rate through the PEM to the cathode side than methanol due
to the reduction in diffusion rates for these larger molecules.
Also useful as freezing point depressants are liquid glycols with a
carbon content greater than two carbon atoms in the molecule such
as, but not limited to, ethanediol, propandiol, butandiol,
pentandiol, hexandiol, heptandiol and octandiol, including any
normal, branched aliphatic or cyclic structural isomers, or
mixtures thereof. An anti-freeze mixture that has a water/alcohol
ratio, or a water/glycol ratio that ensures that the mixture
remains liquid at temperatures in the range +80.degree. C. to
-40.degree. C. and prevents freezing of the solution at
temperatures in the range 0 to -50.degree. C. is preferred. The
concentration of alcohol or glycol is in the range of from 5 to 75
vol %.
[0022] FIG. 1 shows the freezing curves for various mixtures of
water and alcohols and of water and glycols. Shown therein is the
temperature at which the mixture changes phase from a liquid to a
solid/liquid mixture. While a 40 to 50% by weight composition of
methanol/water mixture freezes at a lower temperature than any of
the other compositions and offers the best protection from
freezing, under normal operational conditions the methanol content
in the methanol/water fuel mixture is approximately 1 molar (about
3% by weight). The higher methanol concentration needed to prevent
freezing at -35.degree. C. strongly promotes methanol cross over by
diffusion and thus increases degradation of the fuel cell materials
significantly. As shown in FIG. 1, the freezing points of the other
compositions are comparable to the water-methanol mixtures and can
protect the fuel cell from freezing in a comparable temperature
range. Since the alternative molecules are larger, they will
diffuse through the membrane at a slower rate, and thus will not be
present on the oxidation side in concentrations comparable to the
corresponding methanol concentration should methanol be present
instead of the alternative freezing point depressants. Thus, the
FPDs are expected to cause less damage to the fuel cell. Further,
the reactive part of the alcohol is the hydroxyl group (OH) which
comprises a smaller amount relative to the remnant carbon chain in
the higher alcohols compared to methanol. Since unwanted side
reactions primarily involve the OH-group, oxygen and the catalyst,
the amount of side reaction is reduced by substituting methanol
with a higher alcohol. Accordingly, alcoholic side reactions at the
cathode are expected to occur at lower rates because of the effects
of molecular structure on the rate of diffusion through the polymer
membrane and because the relative activity of the unwanted side
reaction processes are reduced as the effective availability of OH
groups is correspondingly reduced in a heavier molecule. Glycols,
which have multiple OH-groups which enhance their freezing point
depressant properties compared to mono-functional alcohols, can
also be used as FPDs.
[0023] In an alternative embodiment of the present invention,
undesired side reactions at the cathode are limited by replacement
of residual oxygen in the fuel cell prior to shutting down with an
inert gas such as, but not limited to nitrogen or carbon dioxide
gas. Inert gases nitrogen and carbon dioxide are non-reactive with
alcohol and catalyst, and any deleterious reaction will not occur
at the cathode during dormancy when oxygen is replaced by any of
these inactive gases singly or in a mixture. The inert gas can be
obtained from an external supply. However, preferred is that carbon
dioxide gas generated during cell operation be used as the inert
flushing gas. Carbon dioxide is a preferred gas for this purpose,
since it is produced during operation of the fuel cell and can
easily be extracted and stored in a CO.sub.2 buffer tank obviating
the need for external supply and logistics.
[0024] FIG. 2 provides diagrams of exemplary fuel cell systems
modified to provide for addition of a freezing point depressant
and/or for flushing of the system with an inert gas. FIG. 2a shows
a fuel cell system modified to add a freezing point depressant.
FIG. 2b shows a fuel cell system modified to flush the system with
an inert gas. FIG. 2c shows a fuel cell system modified to add a
freezing point depressant and to flush the system with an inert
gas. As shown in FIGS. 2a-2c, this modified fuel cell system
comprises a fuel cell stack 2, which is supplied with a liquid
methanol/water mixture (typically 3 vol.-% methanol) on the anode
side 3, and air on the cathode side 4. The liquid fuel is
circulated by means of a circulation pump 5, and the partially
depleted methanol/water mixture is returned to the storage tank 6.
Water and methanol is taken from two storage tanks, and mixed in
the proper ratio in order to yield the concentration of methanol
used in fuel cell operation. In addition, as shown in FIGS. 2a and
2c, the fuel cell system may comprise a freezing point depressant
storage tank 7 which supplies the fuel cell with a freezing point
depressant during shut-down. The inert gas carbon dioxide, a
by-product of the reaction of methanol and water in the fuel cell,
is separated from the liquid fuel, and in the exemplary embodiment
depicted in FIG. 2b and 2c is stored in a CO.sub.2 storage tank 8
for later use as an inert flushing gas during fuel-cell shut-down.
The CO.sub.2 necessary is readily available without the need for
replenishment from an external supply as after use the storage can
readily be refilled with the CO.sub.2 generated during operation of
the fuel-cell while excess CO.sub.2 can be released to the
atmosphere. An alternative inert gas to be used is nitrogen.
[0025] In accordance with the present invention a procedure for
shutting down, storing and restarting of a modified fuel cell is as
follows:
[0026] The electrical load on the DMFC is released, bringing the
DMFC to the open circuit state. This step is a conditional
requirement for shutting down the fuel cell. As a second step, the
feeding of methanol fuel to the DMFC is interrupted. As methanol is
capable of diffusing through the polymer electrolyte membrane to
the cathode side of the MEA, it is good practice to lower the
methanol concentration at the electrode. Subsequently, as a third
step, an aqueous solution of a freezing point depressant, such as a
higher alcohol or glycol, is fed into the liquid fuel compartment
and allowed to diffuse through the MEA and replace the
methanol.
[0027] At this point, the DMFC is in a shut down state consuming no
energy. Degrading side-reactions have been eliminated and the fuel
cell can be maintained for protracted periods without noticeable
degradation of the electrode and electrolyte components.
[0028] A further advantage of shutting the fuel cell down in
accordance with the present invention is that during start-up of
the DMFC any freezing point depressants can be consumed by the
power generating process and not by unwanted side reaction upon
re-starting whereupon the inert flushing gas is steadily replaced
by air and the DMFC is subjected to a small electrical load. Heat
is generated during the start-up of the DMFC. Since the power
generating reaction involving the higher alcohols is generally
slower than that of methanol, heating occurs more slowly thus
giving time for more uniform heat distribution through the entire
DMFC thereby reducing the risk of forming hot spots with ensuing
damage. Further, by-products of the higher alcohols such as
aldehydes and ketones can be removed by the air stream passing the
cathode during start-up. These products may be further finally
oxidized to the corresponding water-soluble acids and removed as
entrained aqueous solutions. The amounts of these by-products are
limited and have no significant influence on the DMFC performance.
The use of a glycol containing two OH-groups as the freezing point
depressants produces residual oxidation products having a lighter
mass and which are even more volatile, thus enhancing removal in
the air stream.
[0029] On restarting a small electrical load is applied to the fuel
cell to initiate the power generating reaction between air and
alcohol or glycol and heating of the fuel cell. The fuel cell is
then either actively or passively warmed to a temperature above the
freezing point of the liquid methanol/water fuel mixture in which
the fuel cell can operate. The alcohol or glycol solution is
replaced by the normal methanol/water fuel solution, the inert gas
is replaced with air, and the power consumption can progress to the
output level desired.
[0030] An alternative procedure for shutting down and starting the
DMFC after storage at temperatures below 0.degree. C. comprises:
releasing the electrical load on the DMFC; stopping the airflow;
flushing the cathode side with inert gas until the voltage is
stabilized and nearly zero; flushing and/or filling the anode with
a freeze point depressant; storing the fuel cell at -18.degree. C.
for at least 12 hours at which point the fuel cell can be raised to
room temperature; raising the fuel cell temperature to
18-22.degree. C.; and circulating fuel (3% methanol/water mixture)
and low airflow through the cell and as soon as possible putting on
a small load.
[0031] The following non-limiting examples are provided to further
illustrate the present invention.
EXAMPLES
[0032] Unless otherwise specified a conventional DFMC, such as
described in U.S. Pat. No. 6,800,391, teachings of which are herein
incorporated by reference, and comprised of a single fuel cell was
used in the following examples. Each fuel cell included an
electrode with an active area of 156 cm.sup.2. The anode catalyst
layer was platinum/ruthenium (Pt/Ru) alloy, and the cathode
catalyst layer was platinum (Pt). A DuPont Nafion.RTM. layer was
used as the proton conductive membrane and electron conductive
carbon cloth was used as diffusion layers for both the anode and
cathode. Conductive carbon plates with multiple flow channels were
used as the current collecting plates for the anode flow field
plate and cathode flow field plate. A methanol fuel solution feed
rate was capable of variation in the range 1-40 ml per minute and a
variable dry air feed rate capable of variation in the range of
0-2.5 liter/minute was provided.
Example 1
[0033] The fuel cell was stored at -3.degree. C. after flushing the
cathode with an inert gas and filling the anode side of the fuel
cell with a 10% ethanol/water mixture. After allowing the fuel cell
to warm to ambient room temperature, and filling the fuel cell with
3% methanol/water fuel, and rejecting the initial effluent of 3%
methanol/water fuel, a gradually increasing electrical load was
extracted. The current-voltage response as function of time is
shown in FIG. 3. As shown therein, initially, the current and
voltage response was somewhat erratic. However, the fuel cell
reached both a constant current and a constant voltage after 8
minutes and was stable after 30 minutes, thus demonstrating that
the cell is capable of being stored without damaging electrode
materials and the MEA.
Example 2
[0034] The fuel cell was stored at -20.degree. C. after flushing
the cathode with an inert gas and filling the anode side of the
fuel cell with a 10% ethanol/water mixture. After allowing the fuel
cell to warm to ambient room temperature, and filling the fuel cell
with 3% methanol/water fuel, and rejecting the initial effluent of
3% methanol/water fuel, a gradually increasing electrical load was
extracted. The current-voltage response as function of time is
shown in FIG. 4. As shown therein, initially the current and
voltage response were somewhat unstable and similar to Example 1.
However, the fuel cell reached stability and provided constant
current and constant voltage after 7 minutes. Subsequently, the
load was increased up in steps to the maximum load. After 40
minutes the cell clearly was able to maintain cell voltage output
in continuous operation. Thus, the fuel cell was shown to be
capable of storage below the freezing point of the normal
methanol/water without damage to the membrane electrode assembly
materials and the electrodes.
Example 3
[0035] The fuel cell was flushed with nitrogen as an inert gas and
filled with at 50% propylene glycol mixture and subsequently stored
at -30.degree. C. After warming to room temperature 50% propylene
glycol mixture and filling the fuel cell with 3% methanol/water
fuel, a gradually increasing electrical load was applied. The
current-voltage response as function of time is shown in FIG. 5.
Although an initially unstable current and voltage response is
shown, the fuel cell reached both a constant current and a constant
voltage after 30 minutes and maintained output voltage at full
power. Thus, as shown the fuel cell can be stored and re-activated
without damage to the MEA materials and the electrodes.
* * * * *